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Home , Blueschist, Facies, Granulite, Greenschist, Hornfels, Metamorphic facies

Metamorphic and Metamorphosed Rocks

l V.M. Goldschmidt (1911, 1912a), and metamorphosed pelitic, calcareous, and Metamorphosed Mafic Rocks psammitic in the Oslo region

l Relatively simple assemblages Reading: Winter Chapter 25. (< 6 major minerals) in the inner zones of the aureoles around granitoid intrusives

l Equilibrium mineral assemblage related to

Xbulk

Metamorphic Facies Metamorphic Facies l Pentii Eskola (1914, 1915) Orijärvi, S. l Certain mineral pairs (e.g. + ) Finland were consistently present in rocks of appropriate l Rocks with K- + at Oslo composition, whereas the compositionally contained the compositionally equivalent pair equivalent pair ( + ) was not + at Orijärvi l If two alternative assemblages are X-equivalent, l Eskola: difference must reflect differing we must be able to relate them by a reaction physical conditions l In this case the reaction is simple: l Finnish rocks (more hydrous and lower MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5 volume assemblage) equilibrated at lower En An Di Als temperatures and higher pressures than the Norwegian ones

Metamorphic Facies Metamorphic Facies Oslo: Ksp + Cord l Eskola (1915) developed the concept of Orijärvi: Bi + Mu metamorphic facies: Reaction: “In any or metamorphic formation which has 2 KMg3AlSi 3O10(OH)2 + 6 KAl2AlSi 3O10(OH)2 + 15 SiO2 arrived at a chemical equilibrium through Bt Ms Qtz at constant temperature and = 3 Mg Al Si O + 8 KAlSi O + 8 H O pressure conditions, the mineral composition is 2 4 5 18 3 8 2 controlled only by the chemical composition. We Crd Kfs are led to a general conception which the writer proposes to call metamorphic facies.”

1 Metamorphic Facies Metamorphic Facies Dual basis for the facies concept l Descriptive: relationship between the Xbulk & l Interpretive: the range of temperature and pressure F A fundamental feature of Eskola’s concept conditions represented by each facies F A metamorphic facies is then a set of repeatedly F Eskola aware of the P-T implications and associated metamorphic mineral assemblages correctly deduced the relative temperatures and F If we find a specified assemblage (or better yet, pressures of facies he proposed

a group of compatible assemblages covering a F Can now assign relatively accurate temperature range of compositions) in the field, then a certain and pressure limits to individual facies facies may be assigned to the area

Metamorphic Facies Metamorphic Facies

Eskola (1920) proposed 5 original facies: l In his final account, Eskola (1939) added:

F F

F F -amphibolite

F F - (now called

F Sanidinite )

F ... and changed the name of the hornfels facies to the hornfels facies l Easily defined on the basis of mineral assemblages that develop in mafic rocks

Metamorphic Facies Metamorphic Facies

Temperature Several additional facies types have been proposed. Most notable are: Sanadinite Formation of Facies F

F Epidote- Pyroxene- - Greenschist Amphibolite Amphibolite Hornfels Facies Facies Facies Facies ...resulting from the work of Coombs in the “burial metamorphic” of New Zealand Pressure Granulite Fyfe et al. (1958) also proposed: Facies F -epidote hornfels Glaucophane- Schist Facies Eclogite F hornfels Facies

Fig. 25-1 The metamorphic facies proposed by Eskola and their relative temperature- pressure relationships. After Eskola (1939) Die Entstehung der Gesteine . Julius Springer. Berlin.

2 Metamorphic Facies Metamorphic Facies

l Table 25-1. The definitive mineral assemblages that characterize each facies (for mafic rocks). Fig. 25-2. Table 25-1 . Definitive Mineral Assemblages of Metamorphic Facies Temperature- pressure diagram showing the Facies Definitive Mineral Assemblage in Mafic Rocks generally accepted Zeolite zeolites: especially , wairakite, limits of the various facies used in this Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite) text. Boundaries are approximate and Greenschist chlorite + albite + epidote (or ) + ± gradational. The Amphibolite hornblende + (-) ± “typical” or average continental Granulite orthopyroxene (+ clinopyrixene + plagioclase ± garnet ± geotherm is from Brown and Mussett hornblende) (1993). Winter Blueschist glaucophane + or epidote (+albite ± chlorite) (2001) An Introduction to Eclogite pyrope garnet + omphacitic pyroxene (± ) Igneous and Metamorphic Contact Facies Mineral assemblages in mafic rocks of the facies of contact meta- . Prentice morphism do not differ substantially from that of the corresponding regional facies at higher pressure. Hall. After Spear (1993)

Metamorphic Facies It is convenient to consider metamorphic facies in 4 2) Facies of medium pressure groups: 1) Facies of high pressure F Most metamorphic rocks now exposed belong to the greenschist, amphibolite, or granulite facies F The blueschist and eclogite facies: low molar F The greenschist and amphibolite facies conform to the volume phases under conditions of high pressure “typical” geothermal F Blueschist facies occurs in areas of low T/P gradient gradients, characteristically developed in zones

F are stable under normal geothermal conditions May develop wherever mafic magmas solidify in the deep crust or mantle: crustal chambers or dikes, sub- crustal magmatic underplates, subducted crust that is redistributed into the mantle

Metamorphic Facies Metamorphic Facies 3) Facies of low pressure 4) Facies of low grades F Albite-epidote hornfels, hornblende hornfels, and F Rocks often fail to recrystallize thoroughly at very low pyroxene hornfels facies: contact metamorphic grades, and equilibrium is not always attained terranes and regional terranes with very high geothermal gradient. F Zeolite and prehnite- pumpellyite facies are F Sanidinite facies is rare- limited to thus not always xenoliths in basic represented, and the magmas and the greenschist facies is innermost portions of the lowest grade some contact aureoles developed in many adjacent to hot basic regional terranes intrusives

3 Metamorphic Facies Combine the concepts of , zones, and facies F Examples: “chlorite zone of the greenschist facies,” the “ zone of the amphibolite facies,” or the “cordierite zone of the hornblende hornfels facies,” etc. F Metamorphic maps typically include isograds that define zones and ones that define facies boundaries F Determining a facies or zone is most reliably done when several rocks of varying composition and mineralogy are available

Fig. 25-9. Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. Winter (2001)

Facies Series A traverse up grade through a metamorphic should follow one of several possible metamorphic field gradients (Fig. 21-1), and, if extensive enough, cross through a sequence of facies

Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.

Facies Series l Miyashiro (1961) proposed five facies series, most of them named for a specific representative “type locality” The series were: 1. Contact Facies Series (very low -P) Fig. 25-3. Temperature- 2. Buchan or Abukuma Facies Series (low -P pressure diagram regional) showing the three major 3. Barrovian Facies Series (medium-P types of metamorphic regional) facies series proposed by 4. Sanbagawa Facies Series (high-P, Miyashiro (1973, 1994). Winter moderate-T) (2001) An Introduction to Igneous and 5. Franciscan Facies Series (high-P, low T) Metamorphic Petrology. Prentice Hall.

4 Metamorphism of Mafic Rocks Metamorphism of Mafic Rocks l Mineral changes and associations along T-P gradients Plagioclase:

characteristic of the three facies series l More Ca-rich become progressively F Hydration of original mafic generally required unstable as T lowered F If water unavailable, mafic igneous rocks will remain largely l General correlation between temperature and unaffected, even as associated are completely re- maximum An-content of the stable plagioclase equilibrated F At low metamorphic grades only albite (An0-3) is stable F Coarse-grained intrusives are the least permeable and likely to resist metamorphic changes F In the upper-greenschist facies oligoclase becomes stable. The An-content of plagioclase thus jumps from An1-7 to F Tuffs and graywackes are the most susceptible An17- 20 (across the peristerite solvus) as grade increases F Andesine and more calcic plagioclases are stable in the upper amphibolite and granulite facies

l The excess Ca and Al ® , an epidote mineral, sphene, or , etc., depending on P-T-X

Metamorphism of Mafic Rocks Mafic Assemblages at Low Grades l Clinopyroxene breaks down to a number of mafic l Zeolite and prehnite-pumpellyite facies

minerals, depending on grade. l Do not always occur - typically require unstable l These minerals include chlorite, actinolite, protolith

hornblende, epidote, a metamorphic pyroxene, etc. l Boles and Coombs (1975) showed that l The mafics that form are commonly diagnostic of the metamorphism of tuffs in NZ accompanied by grade and facies substantial chemical changes due to circulating fluids, and that these fluids played an important role in the metamorphic minerals that were stable

l The classic area of burial metamorphism thus has a strong component of hydrothermal metamorphism as well

Mafic Assemblages of the Medium Greenschist, Amphibolite, Granulite P/T Series: Greenschist, Facies l Zeolite and prehnite-pumpellyite facies not present in Amphibolite, and Granulite Facies the Scottish Highlands l The greenschist, amphibolite and granulite facies l Metamorphism of mafic rocks first evident in the constitute the most common facies series of regional greenschist facies, which correlates with the chlorite metamorphism and biotite zones of the associated pelitic rocks l The classical Barrovian series of pelitic zones and the F Typical minerals include chlorite, albite, actinolite, lower-pressure Buchan-Abukuma series are epidote, quartz, and possibly calcite, biotite, or variations on this trend F Chlorite, actinolite, and epidote impart the green color from which the mafic rocks and facies get their name

5 Greenschist, Amphibolite, Granulite Facies Greenschist, Amphibolite, Granulite Facies

l ACF diagram l Greenschist ® amphibolite facies transition l The most involves two major mineralogical changes characteristic mineral assemblage of the 1. Albite ® oligoclase (increased Ca-content greenschist facies is: with temperature across the peristerite gap) chlorite + albite + epidote + actinolite ± 2. Actinolite ® hornblende (amphibole accepts quartz increasing aluminum and alkalis at higher Ts)

l Both transitions occur at approximately the

Fig. 25-6. ACF diagram illustrating same grade, but have different P/T slopes representative mineral assemblages for metabasites in the greenschist facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Greenschist, Amphibolite, Granulite Facies

l ACF diagram

l Typically two-phase Hbl-Plag

l are typically black rocks with up to about 30% white plagioclase

l Like diorites, but differ texturally

l Garnet in more Al-Fe-rich and Ca-poor mafic rocks

l Clinopyroxene in Al-poor-Ca- rich rocks

Fig. 25-7. ACF diagram illustrating representative mineral assemblages for metabasites in the amphibolite Fig. 26-19. Simplified for metamorphosed mafic rocks showing the location of several determined facies. The composition range of common mafic rocks is univariant reactions in the CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system ( “C(N)MASH”). Winter (2001) An shaded. Winter (2001) An Introduction to Igneous and Introduction to Igneous and Metamorphic Petrology. Prentice Hall . Metamorphic Petrology. Prentice Hall.

Greenschist, Amphibolite, Granulite Greenschist, Amphibolite, Granulite Facies Facies o l Amphibolite ® granulite facies ~ 650-700 C l Mafic rocks generally melt at higher

l If aqueous fluid, associated pelitic and quartzo- temperatures

feldspathic rocks (including granitoids) begin to l If water is removed by the earlier melts the melt in this range at low to medium pressures ® remaining mafic rocks may become depleted and melts may become mobilized in water

l As a result not all and quartzo-feldspathic l Hornblende decomposes and orthopyroxene rocks reach the granulite facies + clinopyroxene appear o l This reaction occurs over a T interval > 50 C

6 Greenschist, Amphibolite, Granulite Facies

l Granulite facies characterized by a largely anhydrous mineral assemblage

l Critical meta-basite mineral assemblage is orthopyroxene + clinopyroxene + plagioclase + quartz F Garnet, minor hornblende and/or biotite may be

Fig. 26-19. Simplified petrogenetic grid for metamorphosed mafic rocks showing the present Fig. 25-8. ACF diagram for the granulite facies. The location of several determined univariant reactions in the CaO -MgO-Al 2O 3-SiO2-H 2O-(Na2O) system (“ C(N)MASH”). Winter (2001) An Introduction to Igneous and Metamorphic composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Petrology. Prentice Hall. Metamorphic Petrology. Prentice Hall.

Greenschist, Amphibolite, Granulite Greenschist, Amphibolite, Granulite Facies Facies Origin of granulite facies rocks is complex and 2) are dry

controversial. There is general agreement, however, F Rocks don’t melt due to lack of available water

on two points F Granulite facies terranes represent deeply buried and 1) Granulites represent unusually hot conditions dehydrated roots of the F Fluid inclusions in granulite facies rocks of S. Norway are F Temperatures > 700o C (geothermometry has CO -rich, whereas those in the amphibolite facies rocks yielded some very high temperatures, even in 2 are H2O-rich excess of 1000oC)

F Average geotherm temperatures for granulite facies depths should be in the vicinity of 500oC, suggesting that granulites are the products of crustal thickening and excess heating

Mafic Assemblages of the Low P/T Series: Albite-Epidote Hornfels, Hornblende Hornfels, Pyroxene Hornfels, and Sanidinite Facies

l Mineralogy of low-pressure metabasites not appreciably different from the med.-P facies series

l Albite-epidote hornfels facies correlates with the greenschist facies into which it grades with increasing pressure

l Hornblende hornfels facies correlates with the amphibolite facies, and the pyroxene hornfels and sanidinite facies correlate with the Fig. 25-9. Typical mineral changes that take place in metabasic rocks durin g progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrolo gy. Prentice Hall. granulite facies

7 Mafic Assemblages of the Low P/T Series: Albite-Epidote Hornfels, Hornblende Hornfels, Pyroxene Hornfels, and Sanidinite Facies Facies of contact metamorphism are readily distinguished from those of medium-pressure regional metamorphism on the basis of:

F Fig. 25-2. Metapelites (e.g. andalusite and cordierite) Temperature- pressure F Textures and field relationships diagram showing the generally F Mineral thermobarometry accepted limits of the various facies used in this text. Winter (2001)

Mafic Assemblages of the Low P/T Series: Mafic Assemblages of the High P/T Albite-Epidote Hornfels, Hornblende Hornfels, Pyroxene Hornfels, and Sanidinite Facies Series: Blueschist and Eclogite Facies

l Mafic rocks (not pelites) develop definitive mineral l The innermost zone of most aureoles rarely reaches assemblages under high P/T conditions the pyroxene hornfels facies l High P/T geothermal gradients characterize subduction zones F If the intrusion is hot and dry enough, a narrow l Mafic are easily recognizable by their color, and zone develops in which break down are useful indicators of ancient subduction zones

to orthopyroxene + clinopyroxene + plagioclase + l The great density of eclogites: subducted basaltic oceanic quartz (without garnet), characterizing this facies crust becomes more dense than the surrounding mantle l Sanidinite facies is not evident in basic rocks

Blueschist and Eclogite Facies Blueschist and Eclogite Facies Alternative paths to the blueschist facies l The blueschist facies is characterized in metabasites by the presence of a sodic blue amphibole stable only at high pressures (notably glaucophane, but some solution of crossite or is possible)

Fig. 25-2. Temperature- l The association of glaucophane + lawsonite is pressure diagram showing the generally accepted diagnostic. Crossite is stable to lower pressures, and limits of the various facies used in this text. Winter may extend into transitional zones (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. l Albite breaks down at high pressure by reaction to jadeitic pyroxene + quartz:

NaAlSi 3O8 = NaAlSi 2O6 + SiO2 (reaction 25-3) Ab Jd Qtz

8 Blueschist and Eclogite Facies Blueschist and Eclogite Facies

l Eclogite facies: mafic assemblage

l omphacitic pyroxene + pyrope- garnet

Fig. 25-10. ACF diagram illustrating representative mineral assemblages for metabasites in the blueschist facies. The composition range of common mafic rocks is shaded. Fig. 25-11. ACF diagram illustrating Winter (2001) An Introduction to representative mineral assemblages Igneous and Metamorphic Petrology. for metabasites in the eclogite facies. Prentice Hall. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Pyroxene Chemistry Pressure-Temperature-Time (P-T-t) “Non-quad” Paths l The facies series concept suggests that a traverse up grade NaAlSi O NaFe3+Si O 2 6 2 6 through a metamorphic terrane should follow a metamorphic field gradient, and may cross through a sequence of facies (spatial sequences) 0.8 l Progressive metamorphism: rocks pass through a series of aegirine- mineral assemblages as they continuously equilibrate to Ca / (Ca + Na) increasing metamorphic grade (temporal sequences)

l But does a rock in the upper amphibolite facies, for example, Ca-Tschermack’s pass through the same sequence of mineral assemblages 0.2 molecule CaAl2SiO 6 that are encountered via a traverse up grade to that rock Augite through greenschist facies, etc.? Diopside-Hedenbergite Ca(Mg,Fe)Si2O6

Pressure-Temperature-Time (P-T-t) Pressure-Temperature-Time (P-T-t) Paths Paths Metamorphic P-T-t paths may be addressed by: 1) Observing partial overprints of one mineral l The complete set of T-P conditions that a rock may experience during a metamorphic cycle from burial assemblage upon another to metamorphism (and ) to uplift and F The relict minerals may indicate a portion of either the prograde or retrograde path (or both) depending upon is called a pressure-temperature-time path, or P-T-t when they were created path

9 Pressure-Temperature-Time (P-T-t) Paths

Metamorphic P-T-t paths may be addressed by: 2) Apply geothermometers and geobarometers to the core vs. rim compositions of chemically zoned minerals to document the changing P-T conditions experienced by a rock during their growth

Fig. 25-13a. Chemical zoning profiles across a garnet from the Tauern Window. After Spear (1989)

Pressure-Temperature-Time (P-T-t) Paths Metamorphic P -T-t paths may be addressed by:

l Even under the best of circumstances (1) overprints and (2) geothermobarometry can usually document only a small portion of the full P -T-t path 3) We thus rely on “forward” heat-flow models for various tectonic regimes to compute more complete P -T-t paths, and evaluate them by comparison with the results of the backward methods

Fig. 25-13a. Conventional P- T diagram (pressure increases upward) showing three modeled “clockwise” P-T-t paths computed from the profiles using the method of Selverstone et al. (1984) J. Petrol. , 25, 501-531 and Spear (1989). After Spear (1989) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.

Pressure-Temperature-Time (P-T-t) Paths

l Classic view: regional metamorphism is a result of deep burial or intrusion of hot magmas

l : regional metamorphism is a result of crustal thickening and heat input during orogeny at convergent plate boundaries (not simple burial)

l Heat-flow models have been developed for various regimes, including burial, progressive thrust stacking, crustal doubling by continental collision, and the effects of crustal anatexis and magma migration

F Higher than the normal heat flow is required for typical greenschist-amphibolite medium P/T facies series

F Uplift and erosion has a fundamental effect on the geotherm and must be considered in any complete model of metamorphism

Fig. 25-12. Schematic pressure-temperature-time paths based on heat-flow models. The Al2SiO5 phase diagram and two hypothetical dehydration curves are included. Facies boundaries, and facies series from Figs. 25-2 and 25-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrolo gy. Prentice Hall.

10 Pressure-Temperature-Time (P-T-t) Paths

l Most examples of crustal thickening have the same general looping shape, whether the model assumes homogeneous thickening or thrusting of large masses, conductive heat transfer or additional magmatic rise

l Paths such as (a) are called “clockwise” P -T-t paths in the literature, and are considered to be the norm for regional metamorphism

Fig. 25-12a. Schematic pressure-temperature-time paths based on a crustal thickening heat-flow model.

The Al2SiO5 phase diagram and two hypothetical dehydration curves are inclu ded. Facies boundaries, and facies series from Figs. 25-2 and 25-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Fig. 25-12c. Schematic pressure-temperature-time paths based on a heat-flow model for

Fig. 25-12b. Schematic pressure-temperature-time paths based on a shallow magmatism heat-flow model. The Al2SiO5 some types of granulite facies metamorphism. Facies boundaries, and facies series from phase diagram and two hypothetical dehydration curves are includ ed. Facies boundaries, and facies series from Figs. Figs. 25-2 and 25- 3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. 25-2 and 25-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrolo gy. Prentice Hall. Prentice Hall.

Pressure-Temperature-Time (P-T-t) Paths Pressure-Temperature-Time (P-T-t) Paths

l Broad agreement between the forward (model) and 1. Contrary to the classical treatment of backward (geothermobarometry) techniques regarding P - metamorphism, temperature and pressure do not T-t paths both increase in unison as a single unified l The general form of a path such as (a) therefore probably “metamorphic grade.” represents a typical rock during orogeny and regional metamorphism Their relative magnitudes vary considerably during the process of metamorphism

11 Pressure-Temperature-Time (P-T-t) Paths Pressure-Temperature-Time (P-T-t) Paths 3. Some variations on the cooling-uplift portion of the 2. Pmax and Tmax do not occur at the same time “clockwise” path (a) indicate some surprising F In the usual “clockwise” P -T-t paths, P occurs max circumstances much earlier than T max . F For example, the kyanite ® transition is F T max should represent the maximum grade at which chemical equilibrium is “frozen in” and the generally considered a prograde transition (as in path metamorphic mineral assemblage is developed a1), but path a 2 crosses the kyanite ® sillimanite transition as temperature is decreasing. This may result F This occurs at a pressure well below P , which is max in only minor replacement of kyanite by sillimanite uncertain because a mineral geobarometer should during such a retrograde process record the pressure of T max F “Metamorphic grade” should refer to the temperature

and pressure at T max , because the grade is determined via reference to the equilibrium mineral assemblage

Pressure-Temperature-Time (P-T-t) Paths 3. Some variations on the cooling-uplift portion of the “clockwise” path (a) in Fig. 25-12 indicate some surprising circumstances F If the P -T-t path is steeper than a dehydration reaction curve, it is also possible that a dehydration reaction can occur with decreasing temperature (although this is only likely at low pressures where the dehydration curve slope is low)

Fig. 25-12a. Schematic pressure-temperature-time paths based on a crustal thickening heat-flow model. The Al2SiO5 phase diagram and two hypothetical dehydration curves are inclu ded. Facies boundaries, and facies series from Figs. 25-2 and 25-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Fig. 25-14. A typical Barrovian-type metamorphic field gradient and a series of metamorphic P-T-t paths for rocks found along that gradient in the field. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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